1. Technical Field of the Invention
The present invention relates in general to Micro Electro-Mechanical Systems (MEMS) devices and in particular, to MEMS actuators.
2. Description of Related Art
Micro Electro-Mechanical Systems (MEMS) refers to the integration of mechanical elements, sensors, actuators and electronics on a common silicon substrate through microfabrication technology. For example, the microelectronics are typically fabricated using an integrated circuit (IC) process, while the micromechanical components are fabricated using compatible micromachining processes that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical components. MEMS devices are attractive candidates for use in spectroscopy, profilometry, environmental sensing, refractive index measurements (or material recognition), as well as several other sensor applications, due to their low cost, batch processing ability and compatibility with standard microelectronics. In addition, the small size of MEMS devices enables the integration of equipment incorporating MEMS devices into mobile and hand held devices.
Reliable actuators are critical components for MEMS devices. The two most common types of MEMS actuators are thermal actuators and electrostatic actuators. One example of an electrostatic actuator is a comb-drive actuator. Comb-drives are linear motors that utilize electrostatic forces acting between two metal or silicon combs to cause motion of a MEMS component, such as a micromirror. The electrostatic forces are created when a voltage is applied between the combs causing them to attract. One comb is typically fixed, while the other comb is moveable. Each comb includes a plurality of comb fingers arranged so that the moveable comb fingers slide past the fixed comb fingers until they are interdigitated. A restoring spring returns the moveable comb to its original position when a voltage is no longer present between the combs. The force available from each finger is relatively small, and therefore, comb-drive actuators typically have between 10 and 200 fingers to produce adequate force for a MEMS device.
However, the stable travel range of the comb-drive actuator is limited by electromechanical side instability. When side instability occurs, the movable comb fingers are displaced perpendicular to the direction of motion and come into contact with the fixed comb fingers. As the moveable comb moves toward the fixed comb, the overlap area between the fingers increases and the cross-axis force becomes greater, which causes the fingers to suddenly snap over the sideway. This is attributed to two effects: the displacement in the direction perpendicular to the direction of motion and the induced moment around the center of the restoring spring.
The stable travel range of electrostatic comb-drive actuators depends primarily on finger gap spacing, initial finger overlapping and spring stiffness of suspended springs. Increasing finger gap spacing of an electrostatic comb-drive actuator is the simplest method for increasing the stable travel range. However, this method requires a high driving voltage, which is undesirable in numerous applications.
Several other approaches have been proposed to extend the stable travel range. For example, one approach uses tilted folded-beam suspended springs instead of straight springs to shift the spring constant in the y direction. The corresponding spring constant in the y-direction changes from:
                                          200            ⁢            EI                                3            ⁢                          x              2                        ⁢            L                          ⁢                                  ⁢        to                            (                  Equation          ⁢                                          ⁢          1                )                                                      600            ⁢            EI                                                              (                                                      3                    ⁢                    x                                    -                                      5                    ⁢                    d                                                  )                            2                        ⁢            L                          ,                            (                  Equation          ⁢                                          ⁢          2                )            where E is Young's modulus, I is the inertia moment of the mechanical spring, d is the projection of the suspended beam length along the x-direction and L is the suspended beam length.
Another approach to extend the stable traveling range includes adjusting the length of the individual comb fingers and utilizing pre-bent suspended springs. More recently, another approach utilizes a second comb electrode to extend the stable traveling range. This approach shifted Ke (the electrostatic negative spring constant) to extend the displacement of the electrostatic comb-drive actuator. Although each of the above approaches can potentially extend the stable traveling range, extra driving voltage is required to achieve this.